Restriction of Selection Indexes

A selection index may be restricted by requiring that the genetic changes it produces in one or more traits equal zero or some predetermined value, or are of specified sign. Retaining the equations of constraint explicitly in the system simplifies the index equations, facilitates the calculation of the 'genetic cost' of each restriction, and gives the economic value needed to stabilise any trait under selection with an unrestricted index. The effect of gradual restriction of a particular trait on the index weighting factors and on the rate of gain for the aggregate genotype and its constituent traits can be readily plotted, and was nearly linear in several cases examined.